This invention relates generally to the art of vehicle detection and in particular to a device for detecting and distinguishing motor vehicles from other vehicles.
A wide variety of vehicle detectors have been available in the past, including such devices as optical photocell systems, direct pressure responsive devices, seismic signal sensors, and acoustic or sonic signal transducers. Of these, some provide adequate response only if they are carefully placed directly within the roadway over which pass the vehicles to be detected. For example, photocell systems must be carefully placed so that a light beam produced by a source mounted on one side of the road directly impinges upon a photocell mounted on the other side of the road. Similarly, devices which respond to the direct pressure exerted by the wheels of passing motor vehicles are presently available but these also require direct placement in the roads over which the motor vehicles pass. The shortcomings of such deployment techniques in enemy controlled areas are obvious.
Other types of sensors, such as seismic detectors, are capable of somewhat more flexible emplacement but still must be implanted in the ground in the immediate area of a particular roadway to provide accurate detection of the passage of vehicles thereon. Thus, optical, direct pressure, and seismic sensors require careful placement, and, therefore, are not suitable for rapid deployment in remote or hostile areas by such desirable methods as dropping from aircraft in the approximate vicinity of suspected roadways.
Similarly, the sonic and acoustic sensing devices presently available, while not requiring the localized emplacement of the aforedescribed sensors, have been found unsuitable in that they lack the capability of distinguishing among acoustic signals emanating from very similar sources. For example, previously available sensors are generally incapable of distinguishing the sounds generated by motor vehicles from those produced by piston powered boats or airplanes passing in the vicinity.